1. Field of the Invention
The present invention relates to a hybrid vehicle including both an engine and an electric motor as drive sources for traveling of the vehicle, and more particularly, to a hybrid vehicle further including an NOx adsorbing device provided in an exhaust passage of the engine.
2. Description of the Related Art
An engine including an NOx adsorbing device provided in an exhaust passage is known from Japanese Patent No. 2600492, in which NOx generated when the engine is operated at a lean air-fuel ratio is adsorbed to the NOx adsorbing device, and when the adsorbing ability of the NOx adsorbing device reaches a saturated level, the air-fuel ratio is temporarily enrichened to restore the adsorbing ability of the NOx adsorbing device. However, when the air-fuel ratio is temporarily enrichened, there arises a problem that a torque shock is generated due to an increase in output torque from the engine.
It is disclosed in Japanese Patent Application Laid-open No. 6-108824, that the variation in output torque from the engine due to the enriching of the air-fuel ratio is moderated by reducing the output torque by any of the following techniques (1) to (3):
(1) Retarding of engine ignition timing; PA1 (2) Fully closing the EACV (an electronic air control valve); or PA1 (3) Fully opening the EGR (an exhaust gas re-circulating device).
However, in the technique for retarding the ignition timing and the technique using the EACV, there is a problem that the amount of decrease in output torque from the engine is not sufficient and, for this reason, the variation in output torque from the engine due to the enrichening of the air-fuel ratio cannot be sufficiently moderated. The technique using the EACV and the technique using the EGR suffer from a problem in that the generation of the torque shock cannot be sufficiently alleviated, because there is a time lag corresponding to a time for which air or an exhaust gas flows when the output torque from the engine is reduced.
The present invention has been accomplished with the above circumstance in view, and it is an object of the present invention to effectively alleviate the torque shock generated during rich-spiking of the air-fuel ratio in a hybrid vehicle including an engine and an electric motor as drive sources for traveling of the vehicle.